Weegy: Because what defines an element are the number of protons and not the number of neutrons. Since protons also carry positive charge, they also determine the number of electrons needed to neutralise the atom's positive charge. [ The number of electrons and their arrangement relate to the chemical properties of the atom and it's related compounds.
Since isotopes of an element all have the same number of protons as the 'normal' element, they have the same number of electrons as well, therefore the same chemical properties.
Isotopes of elements have the same number of protons, but a different number of neutrons, and so a different atomic mass. ] (More)